Zinc dialkyldithiophosphates (ZDDP) have been added to lubricant compositions for decades due to their low cost, and ability to act as a multifunctional additive-possessing antiwear, and antioxidant properties. Despite the great benefit of these materials, it is well known that the sulfur and phosphorus from these compounds volatilize and pass through the exhaust systems of internal combustion engines where they inevitably poison catalytic converters.
One method to minimize the amount of sulfur and phosphorus reaching the catalytic converter is to reduce the amount of ZDDP present in the lubricant composition. In doing so, it is necessary to increase the amount of other additives to compensate for the antioxidant and antiwear functionality lost by the decrease or removal of ZDDP.
There are a multitude of examples in the patent literature where ZDDP concentrations are either reduced or eliminated. In these examples, the term “low phosphorus” is often used, but as this is a relative term, it may include phosphorus levels as high as 1000 ppm P. Therefore it is necessary in the following discussion to define a “low phosphorus” composition as any lubricant composition containing a phosphorus level below 600 ppm P. This is a minimum performance standard requirement, as set for an ILSAC GF-4 passenger car motor oil. The terms “zero phosphorus”, “no phosphorus”, or “phosphorus free” are defined herein as phosphorus concentrations less than or equal to 10 ppm P.
For the purpose of this discussion, a “low sulfur” composition is defined as any lubricant composition containing a sulfur level below 500 ppm S. This is defined as the minimum sulfur content required of an ILSAC GF-4 SAE 0W or SAE 5W grade motor oil.
Zero phosphorus lubricant formulations which maintain an acceptable level of wear have been demonstrated through the addition of a complex blend of phosphorus free antiwear additives, ashless friction reducers, extreme pressure additives, antioxidants, detergents and polymeric viscosity modifiers and flow improvers, as exemplified in U.S. Pat. Nos. 5,346,635, and 5,439,605. These examples are not low sulfur formulations, as defined above.
Low phosphorus or zero phosphorus lubricant formulations having acceptable wear have been demonstrated with the addition of detergents, as exemplified in U.S. Pat. Nos. 6,159,911, 6,784,143, and U.S. Pat. App. No. 2007/0049507. These examples are not low sulfur formulations, as defined above.
Low phosphorus lubricant formulations having acceptable wear have been demonstrated with the addition of an organomolybdenum dithiocarbamate compound, as exemplified in U.S. Pat. Nos. 6,500,786, and 6,852,679. These are not low sulfur formulations, as defined above.
Low phosphorus lubricant formulations having acceptable wear have been demonstrated with the addition of ashless compounds, such as sulfurized olefins (U.S. Pat. Nos. 4,330,420, and 6,884,855) and dithiocarbamates (U.S. Pat. Nos. 4,758,362, 6,852,680, and 7,160,845). These are not low sulfur formulations, as defined above.
Low phosphorus lubricant formulations having acceptable wear have been demonstrated using borated succinimide dispersants, as exemplified by U.S. Pat. No. 7,122,508. These are not low sulfur formulations, as defined above.
A zero phosphorous, low sulfur lubricant composition and method of use is described in U.S. Pat. No. 6,588,393 where a continuously fresh stream of lubricant is added to a running engine and the equivalent amount of used oil is removed and combined with the fuel stream. This is a unique system designed to reduce NOx emissions. Properties, such as wear, and friction are not considered.
It has now been discovered that a phosphorus and sulfur free organic tungsten complex may be employed in a lubricant composition containing no phosphorus and no sulfur. The organotungstate provides excellent antiwear and limits oxidation and corrosion. The organotungstate also proves effective in lubricant compositions where phosphorus and sulfur are present.
The patent literature contains several examples of tungsten being used in lubricant compositions. For instance, the use of ammonium tungstate salts, also referred to as oxytungstate salts, in aqueous environments is known. In particular, U.S. Pat. Nos. 4,626,367, and 4,816,303, and European Patent No. 2,044,186 disclose how simple alkali earth tungsten salts may be employed as aqueous corrosion inhibitors. Tungstate salts have also been used as antioxidants in aqueous tin electroplating systems, as disclosed in U.S. Pat. Nos. 5,378,347, and 7,151,049.
Tungsten salts have been employed in nonaqueous base lubricant compositions as well. Simple inorganic salts of tungsten may be dispersed in grease and oil compositions, to be used as corrosion inhibitors, as exemplified in U.S. Pat. Nos. 6,010,984, 6,010,985, 6,017,857, 6,316,392, 6,331,509, 6,534,450, 6,632,781, 6,737,387, 6,858,160, and 7,265,080.
Oil and grease soluble tungstate salts have been prepared, most commonly through the incorporation of alkylammonium cations as exemplified in U.S. Pat. Nos. 4,298,485, 7,335,625, 7,858,565, 7,879,777, and 7,820,602 describe lubricant compositions containing alkylammonium polyoxotungstates as antioxidant agents. Further, U.S. Pat. Nos. 3,290,245 and 4,298,485 disclose the use of an oil soluble alkylammonium polyoxotungstate salt as a detergent and dispersant of cold sludge, and as a friction reducer when combined with a sulfur source (i.e. ZDDP).
U.S. Pat. No. 2,795,549 discloses the potential use of oil soluble ammonium 4-t-butyl catechol vanadate and tungstate salts, as copper and lead corrosion inhibitors. More complex salts of tungsten may also be employed in lubricant compositions, such as the polycarboxylate salts of U.S. Pat. Nos. 5,321,146, 5,641,472, and 5,629,435. The tungsten complexes used in the lubricating composition of this invention are not tungstate salts, and are therefore not analogous to tungstate salts.
There are several references in the patent literature to other organotungsten compounds being used in lubricant compositions, which are not tungstate salts. For example, tungsten carboxylates such as that claimed, but not taught, in U.S. Pat. No. 4,824,611, could be employed in non-aqueous lubricant systems. U.S. Pat. No. 3,234,129 discloses a lubricating composition containing either an oil soluble diarene tungsten tricarbonyl, arene tungsten, or a dichlorotetranaphthyloxy tungsten which is effective as an antiwear additive, but only when combined with ZDDP. U.S. Pat. No. 6,211,123 describes the use of an oil soluble trinuclear thiotungstate for antiwear, antioxidant and friction control in a lubricant composition. U.S. Pat. Nos. 4,529,526, and 4,171,558 claim lubricating composition containing a zinc, molybdenum or tungsten dialkyldithiophosphate as antiwear agents, but only teach zinc and molybdenum. U.S. Pat. Nos. 3,068,259 and 3,193,500 described an extreme pressure lubricant containing a tungsten dialkyldithiophosphate which must be prepared from tungsten pentachloride.
Tungsten dithiocarbamates and their use in lubricant compositions are also known. U.S. Pat. No. 4,846,983 declares the synthesis of a tungsten dithiocarbamate from WO3 but contains neither data confirming that the complex was formed, nor does it exemplify a lubricant composition containing the organic tungsten complex according to an embodiment of the present invention. Other preparations of tungsten dithiocarbamates, such as those in U.S. Pat. Nos. 5,308,519, and 6,211,123 and World Patent Application WO2004/043910, teach the synthesis of various tungstates by either the use of expensive starting materials and reagents, or by producing tungsten dithiocarbamates in yields which are not viable on a commercial scale.
It has now been unexpectedly discovered that a phosphorus and sulfur free organic tungsten complex according to embodiments of the present invention imparts antiwear, corrosion, and antioxidancy properties upon a lubricating composition. These characteristics are maintained even in lubricant compositions containing reduced phosphorus and sulfur levels.